cmd/api: skip TestIssue29837 when -short is set

cmd/api: skip TestIssue29837 when -short is set

TestIssue29837 takes a long time to run, ~150s on my laptop
and ~40s on CI builders.

While here, warm up the go list cache in parallel, which reduces the
test time by ~10%.

Change-Id: Ib8d45b086453ee03e6c9f3f070d6f6b0d324bfd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/502095
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>